The present invention relates to an improved piston for disc brakes, such as vehicular disc brakes. More in particular, the present invention relates to an improved plastic brake piston having a metallic cap positioned on the face of the piston which is adapted to engage the brake shoe or brake shoe attaching means, such as a clip. The cap provides a protective cover for the plastic piston face nearest the braking means and also provides a means for retaining the brake shoe or brake shoe clip, and may also provide a means for retaining a dust boot. Also, the metallic cap can provide a means for shielding the dust boot from the adverse affects of heat.
Although the present invention will be described in terms of disc brakes, particularly intended for motor vehicles, it will be understood that the present improved piston is equally adaptable to piston brakes utilized on railroad vehicles and industrial applications such as engines.
One type of brake assembly presently in use in motor vehicle disc brakes is the opposed piston type. In such type, a piston is positioned adjacent each opposite side of a brake disc and forces its respective brake shoe or lining against the disc to impart a braking action. Other types of brake assemblies to which the present invention is equally useful are those in which a single piston is utilized to move the brake shoe or lining into contact with both sides of the brake disc by utilizing a caliper slide to impart a braking action.
Upon the advent of the use disc brakes in motor vehicles, the pistons used were metallic, usually fabricated of chrome plated steel, and required several time consuming and costly machining operations. More recently, pistons for disc brakes have been fabricated of a plastic material, usually a phenolic resin molding compound. Such pistons may be produced by molding techniques known in the art. Typical plastic pistons are described in U.S. Pat. No. 4,170,926, the teachings of which are hereby incorporated herein by reference. Plastic pistons can be molded with at least one ring bead on the force portion of the mold to create an undercut to assist in withdrawing the piston from the mold when the mold is opened. Up to the present time, such ejector beads have been positioned on the piston to be inconspicuous and non-interfering with the piston operation. It has now been found that the ejector bead may be utilized to provide an improved brake piston.
More recent designs of plastic pistons embody a hollowed cylinder having an internal groove for retaining a brake shoe clip. The groove, by necessity, has to be formed after the molding operation and requires a machining step. Such pistons may also require a groove, typically on the outside wall, to retain a dust boot or cover. The forming of a dust boot groove in the piston also requires machining after molding, which, in turn, adds a further machining step. It has now been found that the ejector bead or beads previously thought a useless necessity can be utilized to eliminate one or more machining steps. In situations where the ejector bead is not used or desired, alternative anchoring means can be provided.